Liquid crystal alignment films are composed of liquid crystal aligning agents. A liquid crystal aligning agent is a material containing polyamic acid molecules, first dispersed in a solvent, then coated onto a substrate to form a film after evaporation of the solvent. Typically the liquid crystal alignment film includes a two-layer structure as in FIG. 1, wherein the upper layer 101 is a light alignment layer, and the lower layer 102 is a conductive layer. Further, 100 is a substrate, 103 are liquid crystal cells, and 104 are polyamic acid having neither a photoreactive group nor a liquid crystal group (the schematic structure of the light alignment film in the related art is shown in FIG. 1)
In the light alignment layer 101, a liquid crystal component is added, which anneals the material for anisotropic effect occurring in photoreaction, improving overall alignment quality. The cumulated charges in the conductive layer 102 can be released on to the substrate side, so the conductive layer 102 is disposed in the lower layer of the alignment film to avoid direct contact with the liquid crystal component. But some conductive layer molecules do not only exist in the lower layer 102, may also migrate into the top layer. Therefore, during annealing the alignment ratio of the light alignment layer 101 is hindered and the alignment deteriorates, resulting in the occurrence of the AC residual image and the reduction of the contrast.
One solution in the related art is to adjust the molecular weights of materials in the upper and lower layers and their surface energy to promote the separation of the upper and lower layers. But this adjustment adds a greater restriction on the molecular designs of each layer, therefore limits the inherent functions of the materials. Moreover, the surface flatness is made worse, further affecting the alignment quality. There is a need to develop a liquid crystal aligning agent to solve the problems of high AC residual image and low contrast of the light alignment layer in the liquid crystal alignment film.